The circuit in figure 2 is OK for very low currents, when larger currents are needed we can add a "pass" transistor. The transistor passes most of the current instead of the zener diode bias resistor. In figure 3 we connect a NPN bi-polar transistor to our basic zener circuit. The current passes from collector (C) to emitter (E) to the load. We will assume R4 equals 180 ohms and D4 is 13 volts. Capacitor C4 is used for additional filtering.